1. Field of the Invention
The present invention relates to a digital scan converter which may be used for a sector scanning ultrasound diagnostic device or the like.
2. Description of the Prior Art
Recently, a sector scanning ultrasound diagnostic device has been developed in which a ultrasound beam scans, in a sector shape, a human body and a reflection signal is utilized for forming a tomographic image. This device radiates the ultrasound beam through a gap between two ribs so that the patients' hearts are mainly examined. The tomographic image is generally displayed on a TV screen. This is mainly because various types of TV equipment can be used, various functions thereof are utilized, and images are easily recorded using a VTR. Further, TV monitors have been utilized in a wide range of fields so that high reliability is guaranteed and the images on the TV monitors are free from flickers. As described above, this device collects data by sector scanning. However, a TV monitor displays images on the screen by linear scanning. Therefore, scan conversion is required. The scan conversion is performed in such a manner that an image signal is temporarily stored in an image memory and the order of specifying addresses for writing and reading out is determined in accordance with the order of data collection and image display, respectively. Generally, the image memory has addresses designating elements in two-dimensional orthogonal coordinates. Picture element data of one frame are respectively stored in corresponding addresses in the memory. Since gaps between respective scanning lines become larger when the scanning lines are apart from the radiation center in sector scanning, addresses in which signals are not written are present in the image memory. Therefore, the resolution of images becomes degraded. In order to prevent this, the scanning width of the deflection angle in sector scanning may be narrowed. However, this results in a long operating time for forming a single tomographic image. Further, diagnosis for organs which move at high speed, such as the heart, cannot be effectively performed.
Alternatively, data may be interpolated when reading out from the image memory. However, it is extremely difficult to decide an address for interpolating the corresponding data. The presence or absence of data is generally determined as to whether the level is "0" or not; however, tomographic image data obtained by the ultrasound diagnostic device include data whose level is "0". For this reason, the interpolation may be performed for an address in which data is already stored. As a result, data become inaccurate, decreasing the reliability of the diagnostic device.